I can't tell whether this is the issue, but Firefox appears to look for a flag in the sessionCheckpoints.json file indicating a successful shutdown:

https://searchfox.org/mozilla-release/source/browser/components/sessionstore/SessionStartup.jsm#223

If you check your file, do you see something like this:

{"profile-after-change":true,"final-ui-startup":true,"sessionstore-windows-restored":true,"quit-application-granted":true,"quit-application":true,"sessionstore-final-state-write-complete":true,"profile-change-net-teardown":true,"profile-change-teardown":true,"profile-before-change":true}

If not perhaps Firefox will skip sessionstore.jsonlz4 and use a file from the sessionstore-backups folder. Does it seem that something from there probably is getting restored?

Hi jscher2000,

That makes sense. It's not uncommon for me to back up a source profile, that's currently in use, with backup software.

What I don't understand is why It never happened before.

I solved the problem by deleting those aforementioned files in the target folder, then launched that profile, in a blank session, exited Firefox, and then replaced the newly created files with the ones from the source profile.

I suppose the same results could have also been accomplished by launching that backup profile and closing it before selecting those files for transfer, which just occurred to me, of course, after your advice.

Thank for the valuable lesson, especially so considering how I go about the business of profile building.